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Messier 42 - M42 - The Great Orion Nebula (Emission/Reflection)

M42, also known as the Great Orion Nebula or Orion Nebula, is the prime deep sky attraction in the constellation of Orion and a showpiece deep sky object. With an apparent magnitude of +4.0, it's easily visible to the naked eye. This emission / reflection nebula and star forming region spans more than a degree of sky and is therefore one of the largest and brightest objects of its type.

Orion is a prominent constellation and one of the most recognizable and familiar sights. Located on the celestial equator, it's visible throughout the World and best seen during the months of December, January and February. The constellation is filled with bright stars, including first magnitude Rigel and Betelgeuse plus a further five second magnitude stars. Three of the second magnitude stars (Mintaka, Alnilam and Alnitak) form the famous belt of Orion. Positioned just 5 degrees south of the belt is the Orion Nebula itself, which is part of the Hunters Sword.

To the naked eye, M42 appears as a soft diffuse glow surrounding the stars of the sword of Orion. When viewed through 10x50 binoculars, it's a prominent feature appearing large and bright. The centre region is obvious, with parts of fainter nebulosity extending outwards towards the east and west in the shape of two wings. Two prominent bright stars, embedded within the nebula, are easily visible at the heart of M42. They form the famous multiple star, Theta1 Orionis, commonly known as the Trapezium. This grouping, which is one of the most observed multiple star systems consists of four bright members in the shape of a trapezoid, plus a few fainter stars. An 80mm (3.1-inch) telescope at low/medium power easily splits the Trapezium into its main components. Combined with the surrounding nebula, it's a fantastic view. The four brightest stars of the Trapezium have magnitudes of +5.1(C), +6.7(D), +6.7->7.7(A) and +8.0->8.7(B) respectively. Unusually, they are lettered in order of right ascension instead of magnitude. A challenge for observers of the Trapezium is to spot two of the fainter members, 11th mag. stars E and F. They can be observed with apertures of 80mm (3.1 inches) on nights of good seeing, but much easier with scopes of the order of 150mm (6 inches) or greater.

Large telescopes reveal more intrigue details in M42. The view through a 200mm (8-inch) telescope is superb. At low magnifications, the Orion Nebula fills the field of view with significant amounts of structural detail, such as twists and wisps of cloud formations visible. The Trapezium is very evident and bright. Visually the Orion Nebula can often exhibit a green hue but photographically it appears mostly red.

The discovery of the Orion Nebula is generally credited to French astronomer Nicolas-Claude Fabri de Peiresc. He recorded it on November 26, 1610. It was then independently located by Johann Baptist Cysat in 1611. Surprisingly, neither Ptolemy's Almagest nor Al Sufi's Book of Fixed Stars noted the nebula, even though they both listed patches of nebulosity elsewhere in the sky. Galileo Galilei made telescopic observations of the surrounding region in 1610 and 1617, but also failed to notice M42. However, he did discover the Trapezium on February 4, 1617. This has led to some to speculate that there has since been a flare-up of the illuminating stars, increasing the brightness of the nebula.

The Orion Nebula is a spectacular deep sky object and one of the most famous of all. It's visible to the naked eye as a faint haze, a wonderful sight in binoculars and spectacular through telescopes. You are looking at a stellar nursery where stars are been born. At the heart of the nebula and illuminating the surrounding region is a group of stars known as the Trapezium. This multiple star system consists of four main bright members and is easily resolvable in small scopes.

M42 is located 1,340 light-years from Earth and has a spatial diameter of 24 light-years. Positioned only 8 arc minutes north of M42 is M43, which is also part of the Orion Nebula. It's separated from the main nebula by a dark dust lane.